The shipment of liquid materials by ISO bulk liquid containers started in the late 1960's. Over the past 50 years, the use of these containers has grown exponentially. What started as an efficient way to ship wine and liquor now includes hazardous and nonhazardous materials, for instance everything from crude oil products to the most dangerous PIH (poison inhalation hazard) products shipped around the world every day.
The design of these containers has changed slightly over the years, but what has remained consistent is the inclusion of a manway. The manway can be located on the top, sides, or ends of the container. The manway is used by operations for loading, and off-loading purposes. (A manway is often also referred to as a manlid, and those terms are used interchangeably herein.) For this reason, the manway/manlid can potentially be opened and closed every time a container is sent to, or used by an operator.
A typical manlid has stainless steel clips that hold swing bolts in place for closure. The elements of this design make it a bolted connection. Additional manway/manlid sealing systems also employ bolts in different configurations known to those of skill in the art. Little thought has been given to the gasket design, torque values, or fasteners that will provide the necessary gasket stress required in effectively and reliably sealing this connection. Historically, and currently the tool of choice to tighten a manlid is a hammer, with no thought given to torque values, or a crisscross torque pattern to bring the gasket down evenly on the connection to ensure consistent gasket stress across the manlid. In many cases these welded stainless clips cause the manlid to warp and distort slightly causing a rather uneven gasket groove. The manway collar is a plate rolled to the proper diameter with a welded seam; the collar also has sixteen clips welded to the outside diameter, two clips for each of the eight swing bolts. The manway collar also warps and distorts during fabrication and is not a machined surface. The design and fabrication process leaves a narrow sealing surface area that is typically not flat or perfectly round. Because of this fabrication process, no two manway fixtures are exactly the same and may be/become out of design tolerances with manufacturers' desired measurements after units are entered into service.
A typical manlid is hinged to the manway collar allowing it to be opened and closed as needed. The manlid is secured to the manway by swing bolts (for example 6-10 bolts) that are also designed to allow the cover to be opened and closed as necessary. The manlid is fitted with a gasket to prevent liquid and vapors from being released. Historically these manlid connections have been the weakest, most vulnerable connection on—bulk liquid containers. Over the many years of its use in the industry many different types of gasket materials and designs have been used to try and accomplish a leak free, reliable seal. Although the development and introduction of new materials have had some positive impact, the shape and basic design of the gasket has remained the same, and none has considered the importance of seal tightness or torque retention in this bolted connection. Consequently, the manlid cover connection is still the most vulnerable and troublesome connection on today's bulk liquid containers.
The shape and design of the manlid connection poses challenges in trying to achieve a reliable, sustainable leak free liquid and vapor seal. The shape of the manlid groove causes a soft or ill-fitting gasket to roll towards the inside of the manway collar. The depth of the groove and the short height profile of the manway collar do not allow for full engagement of the two surfaces when the manlid is in the closed position without a gasket installed. The wide width of the manlid groove and the thin width of the manway collar make for a very narrow sealing surface. All of these factors have over the years made this unique bolted connection a trouble spot for leaks in the industry. Considering the multiple vibration cycles these containers are exposed to during transport, the frequency that the covers are opened and closed, the desirability of reusing the gasket, and the poor, uncontrolled assembly procedures and tools that are used, it can be seen that all these detrimental attributes are exacerbated.
Currently in the industry there are several gasket designs and materials commonly being used. All are of a flat or round design shape which allows only one point of gasket contact with the manway sealing surface. The gaskets are made of an elastomer, PTFE, or a combination of the two materials. Although each gasket type has been used for many years in the industry, no current design shape or material gasket has taken into consideration all the necessary elements of this flanged bolted connection to provide the industry with a gasket that consistently achieves a complete reliable liquid and vapor seal for the hazardous and lethal commodities being transported in—bulk containers globally today. The relatively wide manlid gasket groove and narrow sealing surface of the manway collar allows the collar to embed itself into the flat or round surface of the gasket designs used today. By doing so the narrow collar will deform, split, or delaminate the gasket's sealing surface. Operators open and close the manlid often to load, or off-load this equipment. The flat or round design of currently used gaskets, do not allow the gasket to capture the collar in the exact relative location each time to ensure a repeatable reliable seal. Instead, the sealing surface on the collar wanders and tries to seal itself over the deformations, splits, and delaminations caused by the previous closure. This creates leak paths and makes it difficult if not impossible to reseal, making the gasket useless after only one or two uses. Additionally, many users of the—bulk containers are located in remote areas or areas where parts are not easily accessible.
Existing types of gaskets including the following:
1) White Nitrile Buna (N)
This gasket has been around in one form or another for many years. The biggest deterrent of this elastomer is its low chemical resistance, its low heat tolerance, its low maximum compressive stress limits and its poor torque retention properties. The narrow sealing surface on the manway collar and flat gasket surface allows for only a small one point of contact sealing surface. The creep and cold flow properties of rubber cause the connection to relax and lose the gasket stress necessary to seal the connection. When tightened over and over to maintain gasket stress the manway collar embeds itself deeper and deeper into the gasket surface causing deformation and splits. When this occurs the gasket is damaged and successful re-use is very unlikely. In addition, this gasket is difficult to install, and very difficult to remove, and has limited dual side use capabilities.
2) EPDM
Similar to sweet white rubber, this gasket also has low chemical resistance properties, low heat tolerance, and low torque retention properties. Its color (black) and lack of markings make it very difficult to identify from other black rubber gaskets. The flat design shape of this gasket allows for only one point of contact with the narrow sealing surface of the manway collar. The flat surface allows the manway collar's narrow sealing surface to wander, not providing a reliable, repeatable sealing area. The creep and cold flow properties of the material cause the gasket to relax, and lose the gasket stress necessary for a sustainable connection. When tightened more and more in the attempt to get the connection to seal, the narrow manway collar embeds itself deeper into the pliable gasket material causing deformation and splitting of the material. When this occurs, the gasket is destroyed and any successful re-use is unlikely. This gasket is fairly easy to install, but very difficult to remove after use, and has limited dual side use capabilities.
3) Hollow Core PTFE
The hollow core PTFE gasket is joined on one end with a smaller diameter tube that inserts into the other end allowing it to be used on multiple size covers. This gap allows a pathway for liquid and vapor to escape. The PTFE material makes the gasket chemically resistant; however, this gasket is not user friendly. It is hard to install and very difficult to remove. If not properly cut and installed, the leak potential is exacerbated. The round shape and hollow design of the gasket, coupled with the wide manlid gasket groove, and the narrow seating surface of the manway collar make it difficult to get a reliable liquid and vapor seal. This gasket provides only one point of contact with the narrow manway collar sealing surface, and provides no spring back, or live action to ensure torque retention during the vibration cycles seen during transport. The manway collar sealing surface embeds itself into the gasket, and the hollow core causes the gasket to collapse upon compression. This collapse may cause the gasket to split creating potential leak paths. The design of the manway does not allow for full engagement of the manway collar into the manlid groove. This means that when fully closed, there is a gap between the top of the manway collar and the bottom of the manlid groove. With the hollow core design of this gasket, and the shorter, narrow sealing surface of the collar, there are points in the connection that do not reach the gasket stress necessary to provide a reliable sustainable liquid and vapor seal regardless of how much the connection is tightened. The gasket design causes deformation and splitting of the material to occur, destroying the gasket and making any re-use of the gasket unlikely. In addition, the gasket is difficult to install correctly, and it has no dual side use capabilities.
4) PTFE Encapsulated Silicone
This gasket is a PTFE encapsulated elastomeric type. It is a thin flat design that fits into the manlid groove. The flat surface allows the manway collar's narrow sealing surface to wander. The narrow manway collar sealing surface embeds into the gasket which can cause the thin PTFE layer of encapsulation to split creating leak pathways and chemical incompatibility issues. Once split, the commodity is free to attack the elastomer and degrade the gasket. The elastomer core creep and cold flow properties cause relaxation of the connection making re-torque necessary. This gasket is very hard to install correctly at the correct assembly torque range. Once removed the gasket is of no further use, and has no dual side use capabilities.
5) Molded PTFE Enveloped Elastomer
This gasket is a PTFE wrapped elastomeric type. It is a flat design that fits into the manlid groove. The elastomer is wrapped in a thin PTFE layer with seams that extend around both the ID and OD of the gasket at the top and bottom. The flat gasket design allows the narrow manway collar sealing surface to wander across the gasket surface. The narrow sealing surface embeds itself into the gasket deforming the gasket making any reliable re-use of the gasket unlikely. The gasket design does not provide any spring back, or lively action to ensure proper gasket stress during the vibration cycles seen during transport. The narrow manway collar sealing surface wandering across the flat gasket surface, and the collar embedding itself into the gasket material causes the PTFE wrap to delaminate at the corners. Once this delamination occurs, the commodity is free to attack and degrade the elastomer creating leak pathways, destroying the gasket, and minimizing any successful re-use. The gasket is fairly easy to install, but very difficult to remove, and has no dual side use capabilities.